The dominant method for writing photo-induced gratings in optical fibers is side-writing with ultraviolet (UV) light through the fiber cladding. An optical fiber having a photosensitive glass core and a surrounding cladding is exposed to ultraviolet light having an intensity which varies periodically along a length of the fiber. The periodically varying intensity pattern is typically provided by applying a UV beam to an optical phase grating as described in Anderson et al U.S. Pat. No. 5,327,515 issued Jul. 5, 1994 which is incorporated herein by reference. Alternatively, the intensity pattern can be provided by an amplitude mask or by interfering a pair of coherent UV beams as described in W. H. Glenn et al U.S. Pat. No. 4,725,110 issued Feb. 16, 1988, incorporated herein by reference. In each of these conventional techniques, the source of UV light is typically a high intensity Excimer laser.
Surprisingly, the rate-determining step in conventional fiber grating manufacture is not writing the grating but rather removing and subsequently reapplying the protective polymer coating that the fiber was provided at manufacture. These coatings are needed to protect the sensitive fiber from contamination and mechanical damage, but typical coatings significantly absorb UV radiation and interfere with grating formation. Moreover the coating would be damaged by UV laser beams. Thus, an initial step in conventional grating formation is striping the polymer coating, as by soaking the fiber in hot sulfuric acid. A new coating must be applied and cured after the grating is formed. The coating removal and reapplication steps consume more than half the time required to write a grating in the conventional process.